1. Field of the Invention
The present invention relates to a hydraulic regenerative braking system for a vehicle.
2. Background Art
It is well known that hydraulic regenerative systems promise improved efficiency over electric regenerative systems incorporating a battery. Hydraulic regeneration involves using a pump, connected in the vehicle drive train, as a retarding device, and then storing the resulting high pressure fluid in an accumulator. On the subsequent vehicle acceleration, the high pressure fluid from the accumulator is routed to a hydraulic motor and the stored energy is recovered in the form of mechanical work which drives the vehicle forward. A low pressure accumulator acts as a reservoir to make up for fluid volume variations within the high pressure accumulator, and also provides a charge pressure to the inlet side of the pump.
Since the accumulator pressure is determined by a gas precharge level in the accumulator, and by the volume change from additional fluid added, the current method of modulating braking and driving forces in hydraulic regenerative systems has been to incorporate a variable displacement device to operate in concert with the fixed pressure accumulator. Variable displacement hydraulic devices can be efficient, but they are typically bulky, heavy and expensive, and do not package easily in automotive passenger vehicles. In addition, space is often limited in the front of a vehicle, yet, because the front wheels of a vehicle typically support 60% of the vehicle mass, plus whatever weight transfer takes place as a result of the vehicle deceleration, an effective regenerative braking system must incorporate braking on the front wheels.
Thus, the packaging problem is further compounded, since the drive train of front wheel drive vehicles is typically very tightly packaged. This leaves little room to add a variable displacement hydraulic device which can operate as a hydraulic pump during braking and a hydraulic motor during acceleration. Fixed displacement pump-motors, or pump-motors having limited variable displacement, may require less space, but may not provide the functionality required of a regenerative braking system. Therefore, a need exists for a hydraulic regenerative braking system that can conserve space by using fixed displacement, or limited variable displacement, pump-motors.
In addition to the packaging problems discussed above, hydraulic pump motors are often undesirably large as a result of their design. In particular, a hydraulic pump-motor that uses pistons, and has a cam that is disposed outside the pistons, may be too large to incorporate into a regenerative braking system on many vehicles. Therefore, a need exists for a hydraulic pump-motor that includes a cam for actuating the pistons, where the cam is disposed inboard of the pistons, thereby conserving space by providing a smaller pump-motor.